1. Field of the Invention
The present invention relates to the art of stencil printing, and, more particularly, a thermal stencil sheet assembly for use as a stencil plate for conducting the stencil printing by employing a thermal stencil sheet adapted to be thermally perforated.
2. Description of the Prior Art
In the art of thermal stencil printing of perforating a very thin thermo-plastic film to have a set of fine holes which provide a desired image as a whole and extruding ink through the holes, when a relatively less fluidal ink such as an emulsion ink having the "minute value" by spread meter less than "32" is used as proposed in Japanese Patent Publication 54-23601, a relatively thick layer of ink may be provided on a perforated thermal stencil sheet with an ink impermeable sheet being attached thereon, and a large number of prints are obtained by such an assembly of the thermal stencil sheet, the ink layer and the ink impermeable sheet being placed on each printing paper and a printing pressure being applied on the ink impermeable sheet with no supplement of ink between each two successive printings.
As a means suitable for practicing such a stencil printing method a thermal stencil sheet assembly comprising an annular frame made of a cardboard, a thermal stencil sheet laid over a first face of said frame and fixed thereto along an entire peripheral portion thereof, and an ink impermeable sheet laid over a second face opposite to said first face of said frame and fixed thereto at a part of an entire peripheral portion thereof has been proposed in Japanese Utility Model Publication 51-132007, and as a means suitable for conducting the stencil printing by employing such a thermal stencil sheet assembly a stencil printing device has been proposed in Japanese Utility Model Publication 57-15814 and is widely used in recent years under a trademark "Printgocco" to print Christmas and new year cards or the like.
It is also known to use a stencil sheet assembly having an annular frame made of a cardboard and a thermal stencil sheet laid over a first face of said frame and fixed thereto along an entire peripheral portion thereof with no ink impermeable sheet to be laid over a second face opposite to said first face of said frame in such a manner that such an assembly is mounted to a stamp having an ink pad impregnated with ink so that the ink pad is applied to the stencil sheet through an opening of the annular frame and the stencil printing is performed in a stamping manner.
Traditionally, the perforation of a thermal stencil sheet has been done in such a manner that the thermal stencil sheet is placed on an original bearing an image formed of an ink or the like which is rich in an infrared rays absorbing material such as carbon or the like and is capable of generating heat by absorption of infrared rays so that its thermo-plastic film is brought into contact with the image bearing surface of the original and light beams rich in infrared rays are irradiated onto the original through the thermo-plastic film, thus the thermo-plastic film being molten by the heat generated at the image portions. However, in accordance with the developments of the word processors, particularly Japanese word processors, in recent years multi-points matrix typing heads for typing characters in the form of a set of fine points have been developed and come to be widely used, and in accordance therewith it has now become possible to directly perforate a thermal stencil sheet by a thermal multi-points matrix typing head, as proposed in Japanese Patent Laying-open Publication 54-33117.
When a thermal stencil sheet comprising a thermo-plastic film and a multi-perforation supporting sheet as Japanese paper or the like pasted together is directly perforated by such a thermal multi-points matrix typing head, it is desired that the typing head is brought into contact with the thermal stencil sheet from the side of its thermo-plastic film, while on the other hand, in printing, it is desired that the perforated stencil sheet is brought into contact with printing paper also on the side of its thermo-plastic film in order to obtain clear printing results. Therefore, it is desired that the configuration of characters or the like to be formed by the thermal multi-points matrix typing head in directly perforating the thermal stencil sheet is reversed relative to that to be printed in a mirror image relation. In view of this it has been proposed in Japanese Patent Laying-open Publication 60-13664 that in a typewriter employing a thermal multi-points matrix typing head the direction of shifting a carriage supporting the typing head is reversed when the perforation typing is performed for a thermal stencil sheet in contrast to a normal typing on a paper sheet.
It is however relatively difficult to mount a relatively thin and soft thermal stencil sheet around the platen normally used in a typewriter or a word processor for directly perforating it in such a typing device now equipped with the thermal multi-points matrix typing head. A conventional method generally adopted to meet with such a difficulty was to lay a thermal stencil sheet on a normal paper with some appropriate binding of the two sheets by paste or the like at several peripheral portions, if necessary, so that the thermal stencil sheet is mounted around the platen of the typewriter or the like under a reinforcement available by the normal paper. In this respect it has been proposed in Japanese Patent Laying-open Publication 63-56450 to provisionally lay a thermal stencil sheet and a paper one over the other as pasted together along its peripheral portion, while the reinforcing paper is provisionally formed with sewing machine carts along its peripheral edge so that the reinforcing paper can be readily removed along the sewing machine carts so as to leave an annular frame portion as bound with the thermal stencil sheet after the typing perforation has been finished. In this publication it has been further proposed that the thermal stencil sheet thus prepared to have perforations and to have an annular frame made of the paper for reinforcement is pasted to a first face of an annular frame made of a cardboard which is provided with an ink impermeable sheet on a second face opposite to said first face thereof so that there is thus obtained a thermal stencil sheet assembly such as proposed in the above-mentioned Japanese Utility Model Laying-open Publication 51-132007.
Since the thermal stencil sheet is relatively thin and soft, it is desirable that the thermal stencil sheet is combined with a reinforcing paper sheet which lies over the entire surface thereof and is pasted together along its entire peripheral portion as proposed in the above-mentioned Japanese Patent Laying-open Publication 63-56450 for the convenience of the thermal stencil sheet being mounted to or dismounted from a typewriter or a word processor for the thermal typing perforation and for the convenience of mounting the perforated thermal stencil sheet to the printing device. However, in this case it is necessary that the central portion of the reinforcing paper must be removed from the combination of the thermal stencil sheet and the reinforcing paper after the thermal typing perforation has been finished before it is mounted to the printing device.
In view of such inconvenience, as a result of experimental researches the inventors of the present application have confirmed that when a sheet material which is thin but relatively elastic is employed for an annular reinforcing frame attached along the peripheral portion of a thermal stencil sheet, such an originally annular thin reinforcing frame is sufficiently effective for improving the convenience of handling the thermal stencil sheet for mounting it to or dismounting it from a typewriter or a word processor and further for mounting it to a printing device. However, since such a thin annular frame does not substantially increase the thickness and the rigidity of a piece of thermal stencil sheet, manufacturing, packing in and out and general transportation handling of such a thin frame reinforced thermal stencil sheet are still not very convenient.